The invention relates to a multi-channel radio relay system, employing a plurality of frequency channels in which both at the transmitting station and at the receiving station a plurality of adjacent high-frequency channels are combined to form a common, high-frequency group, over a cascade circuit comprising channel branch elements including circulators and band-pass filters.
In accordance with known techniques, the transmission of communication channels in the micro-wave frequency band is based on predetermined frequency plans, in accordance with which the high-frequency groups are divided into individual channels. An essential feature governing the construction of transmission links is that the entire high-frequency group be transmitted across a radio field with only a single transmitting antenna and a single receiving antenna. At the transmitting end, the individual frequency channels are combined over branch circuits and supplied to the transmitting antenna, whereas at the receiving end the individual channels are again separated over branch circuits which, for example, are of a construction similar to those at the transmitting end. If the individual channels were combined and separated in the associated branch circuits without special measures, different transit time characteristics would occur with respect to the individual channels, necessitating the use of substantially differing types of transit time correctors in order to compensate for the different transit times.
German Pat. No. 1,260,562 is directed to at least partially eliminating these difficulties and discloses a radio system having a radio relay link, in which it is assured that the sequence of connection of the individual channels for the receiving-end channel branch cascade is the reverse of that for the associated transmitting-end channel branch cascade. With this arrangement, the number of total reflections and the number of filter flanks which together determine transit time distortions on the transmission path from transmitter to receiver, for each individual channel, is equal to the number of total reflections and the number of active filter flanks of all other channels. In a radio system of this special design, although the transit time distortions of the middle channels in the frequency position are equal to one another, the transit time characteristics are only approximately equal for the edge channels. This creates a problem which is particularly disturbing when it is important to utilize identical transit time correctors at the intermediate frequency level for all channels.
In an effort to eliminate these difficulties, German OS 2,213,962 discloses a radio system having a radio relay link, in which a specific sequence of the individual channel branch elements, assigned to the particular high-frequency channels, is provided and in which the direction of circulation of two circulators is opposite to that of the other circulators. Furthermore, a resonator is provided in such case which in the frequency range from the lowest to the highest frequency channel simulates the transit time characteristics produced by the adjacent channel branch in the frequency position homologously to the middle band frequency of such channel.
As a result of this specific design of a radio system, although a transit time distortion and attenuation distortion which is equal and symmetrical for all channels is achieved, the channel branch elements for the edge channels differ from the other channel branch elements, and as a result it is not possible to employ standard assemblies. Furthermore, the micro-wave resonator and the adjustment necessitated thereby results in a generally undesirably high cost outlay.